I've started my first business with £1,000 and bootstrapped my second tech startup up to proof of concept ahead of raising investment.

I agree with the advice you received; the amount of great business cases that come across investors desks these days will make it difficult for you idea to make it past the post box; but you never know. Sometime, it is simply a case of finding the right person at just the right time. The best thing you can do is take you idea as far as you can yourself and network like a ninja.

If you told us where you are in the world, we could possibly give you some additional practical advice.
For instance, I started my businesses in the UK and there are many other routes available to fund your startup and get a few accolades before approaching investors. Look at government backed startup loans and government grants.

This is very close to home for me as I spent the last decade developing and advising businesses including Fortune 500 companies on competitive and growth strategies.

Ahead of thinking of the channels; that's easy, ask yourself, why?

Why, would a competitor's customer want my product over theirs?

The best exercise you can do in this case is a SWOT analysis:
You don't have to make it overly academic; look at your strengths and list them all. Then do the same with your weaknesses.

Then repeat the process for your competitors; do some research; look at their product quality, pricing, availability etc...

Then analyse the key elements of the value proposition:
> Product/ service differentiation: e.g. in value, in quality, in design, in exclusivity etc...
> Pricing: more or less than competitors; why?
> Channels: availability, access, speed, ease

Then go through the Opportunities and the Threats.
Identify how you could convince their audience that you have a better deal (avoid price wars unless you have enough market share and cash to ride the storm - this rarely ends well)

Then design your strategy; where, what, how, when

Reach out if you need more help; good luck

It largely depends on the amount and the business type.
Investors will like maximum returns for as little risk possible so the earlier you go for funding in your launch plan, the harder it is unless your business plan is so strong that any investors will want to take the punt.

Investors typically will want to see how much you invested yourself and how far you tried to go prior to raising funds.

The best time to approach early stage seed investors in my opinion is when you are market ready and are just starting to show case the proof of concept with early market feedback.
Of course this depends on your business plan; how much growth/ market potential there is, the ROI on offer, the exit and timings around the exit and any risk mitigations you can offer to investors (for instance in the UK, you can get small enterprise investment scheme approved, which provides 75% investment protection).

There are many non equity routes you should look to explore ahead of considering equity; obviously the earlier you offer equity, the more you give away early.

I've gone through a lot of this myself. Pumping money into social network advertising is not (or highly unlikely) to return a strong ROI but it depends on your product and who you want to reach.
It is much more effective when your brand presence has grown enough to create a Halo around your advertising; right at the start, you are going to burn a lot of cash for little results.
The best thing to do is to get the basics right first. Create multiple touchpoint for your business with social media pages (it looks like you have that). Make sure you have a few videos on YouTube, this counts toward SEO. Be active with blogging and content engagement. Publish on LinkedIn, create a blog and start engaging your audience and drive traffic to your site.
If your business wants B2B advertising, check www.arlians.com; it's in BETA but if you sign up now, you can get some level of advertising access for Free (for life)